1. Field of the Invention
The present invention relates to the field of chemical and biochemical reactions. More specifically, the present invention relates to parallel synthesis and assay of a plurality of organic and bio-organic molecules on a substrate surface in accordance with a predetermined spatial distribution pattern. Methods and apparatus of the present invention are useful for preparing and assaying very-large-scale arrays of DNA and RNA oligonucleotides, peptides, oligosacchrides, phospholipids and other biopolymers and biological samples on a substrate surface.
2. Description of the Related Art
Development of modem medicine, agriculture, and materials imposes enormous demands on technological and methodological progress to accelerate sample screening in chemical and biological analysis. Development of parallel processes on a micro-scale is critical to the progress. Many advances have been made in this area using parallel synthesis, robotic spotting, inkjet printing, and microfluidics (Marshall et al., Nature Biotech. 16, 27-31 (1998)). Continued efforts are sought for more reliable, flexible, faster, and inexpensive technologies.
For high-throughput screening applications, a promising approach is the use of molecular microarray (MMA) chips, specifically biochips containing high-density arrays of biopolymers immobilized on solid surfaces. These biochips are becoming powerful tools for exploring molecular genetic and sequence information (Marshall et al., Nature Biotech. 16, 27-31 (1998) and Ramsay, Nature Biotech. 16, 40-44 (1998)). Target molecules have been hybridized to DNA oligonucleotides and cDNA probes on biochips for determining nucleotide sequences, probing multiplex interactions of nucleic acids, identifying gene mutations, monitoring gene expression, and detecting pathogens. Schena, et al., Science 270, 467-460 (1995); Lockhart et al., Nature Biotech. 14, 1675-1680; Weiler, Nucleic Acids Res. 25, 2792-2799 (1997); de Saizieu et al., Nature Biotech. 16, 45-48; Drmanc et al., Nature Biotech. 16, 54-58. The continued development of biochip technology will have a significant impact on the fields of biology, medicine, and clinical diagnosis.
Prior art biochip-fabrication includes direct on-chip synthesis (making several sequences at a time) using inkjets, direct on-chip parallel synthesis (making the whole array of sequences simultaneously) using photolithography, and immobilization of a library of pre-synthesized molecules using robotic spotting (Ramsay, Nature Biotech. 16, 40-44 (1998)). Light-directed on-chip parallel synthesis has been used in the fabrication of very-large-scale oligonucleotide arrays with up to one million sequences on a single chip.
Two major methods have been disclosed: synthesis using photolabile-group protected monomers (Pirrung et al., U.S. Pat. No. 5,143,854 (1992); Fodor et al., U.S. Pat. No. 5,424,186 (1995)) and synthesis using chemical amplification chemistry (Beecher et al., PCT Publication No. WO 98/20967 (1997)). Both methods involve repetitive steps of deprotection, monomer coupling, oxidation, and capping. Photomasks are used to achieve selective light exposure in predetermined areas of a solid substrate surface, on which oligonucleotide arrays are synthesized.
For the synthesis process involving photolabile-protecting groups, the photolabile-protecting groups are cleaved from growing oligonucleotide molecules in illuminated surface areas while in non-illuminated surface areas the protecting groups on oligonucleotide molecules are not affected. The substrate surface is subsequently contacted with a solution containing monomers having a unprotected first reactive center and a second reactive center protected by a photolabile-protecting group. In the illuminated surface areas, monomers couple via the unprotected first reactive center with the deprotected oligonucleotide molecules. However, in the non-illuminated surface areas oligonucleotides remain protected with the photolabile-protecting groups and, therefore, no coupling reaction takes place. The resulting oligonucleotide molecules after the coupling are protected by photolabile protecting groups on the second reactive center of the monomer. Therefore, one can continue the above photo-activated chain propagation reaction until all desired oligonucleotides are synthesized.
For the synthesis process involving chemical amplification chemistry, a planer substrate surface is linked with oligonucleotide molecules (through appropriate linkers) and is coated with a thin (a few micrometers) polymer or photoresist layer on top of the oligonucleotide molecules. The free end of each oligonucleotide molecule is protected with an acid labile group. The polymer/photoresist layer contains a photo-acid precursor and an ester (an enhancer), which, in the presence of H+, dissociates and forms an acid. During a synthesis process, acids are produced in illuminated surface areas within the polymer/photoresist layer and acid-labile protecting groups on the ends of the oligonucleotide molecules are cleaved. The polymer/photoresist layer is then stripped using a solvent or a stripping solution to expose the oligonucleotide molecules below. The substrate surface is then contacted with a solution containing monomers having a reactive center protected by an acid-labile protecting group. The monomers couple via the unprotected first reactive center only with the deprotected oligonucleotide molecules in the illuminated areas. In the non-illuminated areas, oligonucleotide molecules still have their protection groups on and, therefore, do not participate in coupling reaction. The substrate is then coated with a photo-acid-precursor containing polymer/photoresist again. The illumination, deprotection, coupling, and polymer/photoresist coating steps are repeated until desired oligonucleotides are obtained.
There are significant drawbacks in the method involving photolabile-protecting groups: (a) the chemistry used is non-conventional and the entire process is extremely complicated; and (b) the technique suffer from low sequence fidelity due to chemistry complications.
The method of using chemical amplification chemistry has its limitations as well: (a) The method requires application of a polymer/photoresist layer and is not suitable for reactions performed in solutions routinely used in chemical and biochemical reactions since there is no measure provided for separating sites of reaction on a solid surface. (b) In certain circumstances, destructive chemical conditions required for pre- and post-heating and stripping the polymer/photoresist layer cause the decomposition of oligonucleotides on solid surfaces. (c) The entire process is labor intensive and difficult to automate due to the requirement for many cycles (up to 80 cycles if 20-mers are synthesized!) of photoresist coating, heating, alignment, light exposure and stripping. (d) The method is not applicable to a broad range of biochemical reactions or biological samples to which a photo-generated reagent is applied since embedding of biological samples in a polymer/photoresist layer may be prohibitive.
Additional limitations are linked to the use of photomasks in the above two methods: (a) Setup for making a new chip is very expensive due to a large number of photomasks that have to be made. (b) Photolithography equipment is expensive and, therefore, can not be accessed by many interested users. (c) Photolithography processes have to be conducted in an expensive cleanroom facility and require trained technical personnel. (d) The entire process is complicated and difficult to automate. These limitations undermine the applications of oligonucleotide chips and the development of the various MMA-chips.
Therefore, there is a genuine need for the development of chemical methods and synthesis apparatus that are simple, versatile, cost-effective, easy to operate, and that can afford molecular arrays of improved purity.